In a power conversion system such as a variable speed, constant frequency (VSCF) power generating system, a brushless, three-phase synchronous generator operates in a generating mode to convert variable-speed motive power supplied by a prime mover into variable-frequency AC power. The variable-frequency power is rectified and provided over a DC link to a controllable static inverter. The inverter is operated to produce constant frequency AC power, which is then supplied over a load bus to one more loads.
As is known, a generator can also be operated as a motor in a starting mode to convert electrical power supplied by an external AC power source into motive power which may in turn be provided to the prime mover to bring it up to self-sustaining speed. In the case of a brushless, synchronous generator including a permanent magnet generator (PMG), an exciter portion and a main generator portion mounted on a common shaft, it has been known to provide power at a controlled voltage and frequency to the armature windings of the main generator portion and to provide field current to the main generator portion via the exciter portion so that the motive power may be developed. This has been accomplished in the past, for example, using two separate inverters, one to provide power to the main generator portion armature windings and the other to provide power to the exciter portion. Thereafter, operation in the generating mode may commence whereupon DC power is provided to the exciter field winding.
Dhyanchand, et al., U.S. Pat. Application Ser. No. 07/421,830, filed Oct. 16, 1989, now U.S. Pat. No. 4,947,100 entitled "Power Conversion System With Stepped-waveform DC/AC Converter Having Prime Mover Start Capability" discloses a power conversion system which is operable in generating and starting modes. The converter includes a rectifier bridge which is coupled to the output of a brushless, synchronous generator driven by a prime mover and first through fourth inverter bridges which are coupled to the output of the rectifier. First through fourth primary windings of a summing transformer are coupled to the outputs of the first through fourth inverter bridges, respectively. The windings of each of the first and third sets of primary windings are connected in a wye configuration whereas the windings of each of the second and fourth sets are connected in a delta configuration. The first through fourth inverters are operated to produce 24-step, constant frequency AC power in a secondary winding in the summing transformer. During operation in a starting mode, an external AC power source is coupled to the secondary winding of the summing transformer, the input of the rectifier bridge is coupled to the wye connected primary windings and the outputs of the first and third inverters are coupled to armature windings of the generator. The first and third inverters are controlled to convert the DC power produced by the rectifier into AC power for causing the generator to operate as a motor and thereby develop the required motive starting power.
Dhyanchand, et al., U.S. Pat. Application Ser. No. 07,462,444, filed Oct. 25, 1989, entitled "Power Conversion System With Stepped-waveform DC to AC Converter Having Prime Mover Start Capability", now U.S. Pat. No. 4,968,926 discloses a power conversion system utilizing a generator coupled to a prime mover and operable in generating and starting modes. The system includes a rectifier having an input and an output, first through fourth inverters each having an input and an output wherein the first inverter includes a plurality of switches and a plurality of flyback rectifiers coupled across the switches and a transformer including first through fourth sets of primary windings and a set of secondary windings wherein the first set of primary windings is coupled to the output of the first inverter. The system also includes an interphase transformer having first and second ends and a mid-tap. Contactors are operable in the generating mode to couple armature windings of the generator to the rectifier input, the rectifier output to the inputs of the inverters and the output of the second through fourth inverters to the second through fourth sets of primary windings, respectively. The contactors are operable in the starting mode to couple the source of AC power to the set of secondary windings, the second set of primary windings to the input of the rectifier, the output of the rectifier to the first end of the interphase transformer, the input of the first inverter to the second end of the interphase transformer, the mid-tap of the interphase transformer to the input of the second inverter and the output of the second inverter to the generator armature windings. The inverters are operated so that AC power produced by the generator in the generating mode is converted into DC power by the rectifier and the DC power is converted into 24-step, fixed-frequency AC power in the set of secondary windings. During operation in the starting mode, AC power induced in the first and second sets of primary windings due to application of AC power to the set of secondary windings is converted into DC power by the flyback rectifiers of the first inverter, the rectifier and the interphase transformer and the DC power is converted into AC power at a controlled frequency by the second inverter which is applied to the generator armature windings. The generator is thereby accelerated to produce motive power which is supplied to the prime mover.
While brushless generators have distinct advantages over brush-type generators, the former requires rotating rectifier assemblies and position sensors to permit operation in the starting mode. These components limit reliability and add complexity to the overall system.
Switched reluctance machines do not require rotating rectifier assemblies and position sensors and hence have advantages over brushless generators. Control of such machines is summarized in a paper entitled "Switched Reluctance Drives" by T.J.E. Miller of Glasgow University, filed herewith. It is known that a switched reluctance machine can be operated as either a motor or a generator.